(Shanghai) — Chinese mainland shares logged their biggest drop in five months on Friday, underscoring investors’ jitters over poor trade figures and a growing valuation bubble after two stocks were given a sell rating.

The benchmark Shanghai Composite Index plummeted 4.4% to close at 2,969.86, the largest decline since Oct. 11. The smaller Shenzhen Component Index lost 3.25%.

Stocks of nonbank financial institutions led the slump after two brokerages this week downgraded leading securities firm China Securities Co. Ltd. and state-owned insurer The People’s Insurance Co. (Group) of China Ltd. (PICC) to a sell rating, citing overvaluation concerns. Both companies fell by the 10% daily limit on Friday.

China’s stock markets have done an abrupt about-face since the start of 2019 amid eased Sino-U.S. trade tension and a slew of policy boosts, and are now traded firmly in bull territory. Shares had gained nearly 25% since January before Friday’s dive, spurring worries of a potential bubble.

“A-shares have been crazy. The gap between prices of A shares and H shares has continued to widen,” Chen Jian, an investment consultant with Datong Securities, told Caixin.

For example, the mainland shares of China Securities were trading at a price 3.72 times higher than the company’s Hong Kong shares, as calculated with the same currency. Typically, major Chinese brokerages’ mainland shares trade at a price less than 1.7 times higher than their Hong Kong counterparts.

“Some worrying signs have also emerged. Funds flowing into the mainland markets through the stock connect programs with Hong Kong are starting to leave and (major shareholders) of many companies have shed their holdings by large margins since the Spring Festival. Brokerages, in very rare moves, gave sell ratings to warn investors to appropriately control their positions,” Chen said.

Yin Yue, a strategist with Lianxun Securities, pointed to fears of worsening economic fundamentals as reflected by disappointing trade figures for February, published by the government on Friday, as another trigger of the sell-off.

Exports of goods dropped 20.7% year-on-year in February, the biggest decline in three years, as the trade war with the U.S. took its toll, according to data released by the General Administration of Customs. Imports fell for the third consecutive month, down 5.2% year-on-year in February, accelerating sharply from a 1.5% decline in January.

Yin said the poor figures, although partly distorted by the week-long Lunar New Year holiday last month, pointed to sluggish demand both at home and abroad and increasing urgency for the government to ratchet up infrastructure investment to stabilize economic growth.